Switched-mode power supplies (SMPSs) are used in a variety applications to supply power and power conversion, (e.g., AC to DC conversion or DC to DC conversion) and have various advantages over other types of power supplies. For example, SMPSs are generally smaller, lighter, more efficient, and have more flexibility than linear regulated power supplies, all else being equal. In certain applications a high output current is needed and a plurality of SMPSs, or more accurately a plurality of the DC-DC power converter portions of a plurality of SMPSs, are arranged in parallel to share the high current load. Such an arrangement is referred to as a paralleled droop load share SMPS unit, or simply a droop load share SMPS.
The power converter of a typical SMPS includes an inverting switch that receives high DC input power and converts the high DC input power to intermediate AC power, a transformer that steps the high AC power down to a low (i.e., stepped-down) AC power, and a synchronous rectifier that converts the stepped-down AC power to low DC output power. The high DC input power may be produced by any DC voltage source, e.g., a rectifier that receives AC power from a mains electricity power supply or a DC voltage battery.
When starting up the power converter of an SMPS, switching of the synchronous rectifier is often started as early as possible. However, in some applications a start-up sequence of cycle includes two phases or modes—a ramp-up mode (i.e., diode mode) in which the synchronous rectifier's switches are off, followed by a ramp-in mode in which the synchronous rectifier is active and switching. The ramp-in mode is entered after the output voltage of the power converter has reached a nominal threshold value.
Typically the paralleled power converters in a droop load share SMPS are started up in the same way as when the power converters are not intended to be connected in parallel, i.e., as described above. However, not all power converters have the same start-up time. A power converter that starts slowly can experience current flowing into its output terminal, i.e., reverse currents, when arranged in parallel with an SMPS that starts more quickly. The reverse currents can result in body diode conduction in MOSFETs used in the inverting switch, avalanche breakdown in MOSFETs used in the synchronous rectifier, as well as overuse of snubbers. Eventually such stress on the components can result in failure of the power converter or, alternatively, a higher cost for the power converter due to the use of more expensive components with better performance that can withstand such stresses. Moreover, during the ramp-up mode of the start-up cycle (i.e., when the synchronous rectifier is in diode mode), the difference in start-up times between power converters can result in that only one of the paralleled power converters starts up and the others do not start until the load on the output has increased. In effect, a high output voltage of a quick-starting power converter causes the control circuitry of the slow-starting power converter to misread the slow-starting power converter's output voltage as being higher than it actually is and therefore the control circuitry may prematurely pause the slow-starting power converter's start-up cycle. This situation can result in interruptions or dips in the output voltage of the paralleled power converters when the load changes fast. Also it can result in high reverse currents for longer amounts of time during the ramp-in mode of the synchronous rectification with the attendant stress on the power converter components.